A fetal arrhythmia was detected in a pregnant woman. Fetal Doppler-echocardiography did not reveal cardiac structural alterations; the Doppler flow velocity recordings at the level of the left ventricular outflow tract revealed the continuous succession of three cardiac cycles: a prolonged cycle (456 +/- 6 ms, mean of five measurements), a medium duration cycle (428 +/- 4 ms) and a short cycle (296 +/- 4 ms) after which the prolonged cycle repeated. Identification of the late ventricle filling phase of the atrial systole in the Doppler flow velocity waveform during the long and medium duration cycles, the detection of the systolic movement of the atrial walls preceding contraction of the ventricle walls during the shortest cycle and the verification that summation of the shortest and longest cycles was inferior to twice the intervening cycle, supported the idea that the arrhythmia was produced by frequent atrial premature contractions (one every two sinus beats). The fetal arrhythmia progressively disappeared during pregnancy until delivery without complications, and the newborn's ECG only revealed sporadic premature atrial contractions.